<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    html {
      font-size: 20px;
    }
    #app {
      /* 1rem好像相当于16px */
      /* rem其实是页面的html根元素的字体大小的倍数 */
      width: 20rem;
      height: 15rem;
      border: 1px solid #000;
    }
  </style>
</head>
<body>
  <div id="app">

  </div>
</body>
</html>